================================================================================
TITLE:

feat(ui): Implement State Diff Panel in Interactive Mode - Issue #1006

DESCRIPTION:

Overview

Implements a resizable, scrollable three-column State Diff panel for the
interactive split-screen trace viewer. The panel shows exactly what changed in
ExecutionState.HostState between consecutive execution steps, with colour-coded
additions, removals, and modifications.

Changes

Core Implementation

• StatePanel (internal/ui/widgets/state_panel.go): New diff widget

  • ComputeDiff(prev, curr *ExecutionState) diffs HostState map[string]interface{}
    between two consecutive steps — the native state change structure in the codebase
  • DiffKind enum: DiffAdded (green +), DiffRemoved (red -),
    DiffChanged (yellow ~), DiffSame (dim) — with stable sort priority:
    changed → added → removed → same, alphabetical within groups
  • StatePanel.Lines(width, maxRows int) renders the 3-column table
    (indicator │ Key │ Old Value │ New Value) into a []string slice the
    layout consumes directly
  • Proportional column widths: 30% key, 35% old value, 35% new value
  • SelectUp/Down() with scrollTop tracking for full keyboard scrollability
  • Summary() returns +N -N ~N count string for status bars
  • SetNoColor(bool) for piped/non-TTY output

• Styles (internal/ui/styles.go): New centralised styling module

  • Single source of truth for all ANSI SGR codes used across the ui package
  • Colorize(text, style string) with NoColor global for piped output
  • DefaultBorder struct for consistent border character sets
  • widgets package exports its own Colorize copy to keep the dependency
    graph acyclic (widgets → trace only, not widgets → ui)

• Layout Extension (internal/ui/layout.go): Three-pane support

  • PaneDiff constant added to the Pane iota alongside existing
    PaneTrace and PaneState
  • ShowDiff bool field and ToggleDiff() method — hides diff panel and
    moves focus to PaneState if diff panel had focus when closed
  • MiddleWidth() / RightWidth() — two-pane: middle fills all remaining
    space, right returns 0; three-pane: remaining split 50/50
  • Render() signature extended to Render(leftLines, middleLines, rightLines []string);
    top/bottom borders extend automatically when ShowDiff is true
  • SplitRatio adjusted from 0.5 → 0.4 to give equal breathing room across
    three panes at typical terminal widths

• TraceView Wiring (internal/ui/trace_view.go): Diff panel integration

  • diffPanel *widgets.StatePanel field added to TraceView
  • NewTraceView now accepts etrace *trace.ExecutionTrace (nil-safe) —
    pass nil to disable the diff panel without breaking existing callers
  • refreshDiff() looks up etrace.States[currentStep] and
    States[currentStep-1], passes both to diffPanel.SetStates(prev, curr);
    step 0 passes prev=nil so all keys appear as DiffAdded
  • refreshDiff() called alongside refreshState() on every tree navigation
    event so the diff panel always reflects the selected step
  • HandleKey routes KeyDiff to ToggleDiff() and dispatches PaneDiff
    Up/Down events to diffPanel.SelectUp/Down()

• Key Binding (internal/ui/keys.go): Diff toggle key

  • KeyDiff constant added to the Key iota
  • 'd' / 'D' mapped to KeyDiff in KeyReader.Read()

Integration

Update NewTraceView call site to pass the ExecutionTrace:

// Before:
tv := ui.NewTraceView(root)

// After (pass nil if ExecutionTrace is unavailable):
tv := ui.NewTraceView(root, v.trace)

Toggle the diff panel at runtime with d. Navigate it with ↑/↓ after
switching focus with Tab or →.

Testing

• Diff panel renders correctly at 80, 120, and 220 column widths
• ComputeDiff correctly classifies added, removed, changed, and unchanged keys
• Step 0 (no previous state) shows all keys as DiffAdded
• ToggleDiff hides panel and reassigns focus when PaneDiff was active
• Scrolling and row selection work independently from the trace and state panes
• Summary() returns correct +N -N ~N counts across all diff kinds

Related Issues

Closes #1006

Type of Change

• New feature

Checklist

• Code follows project style guidelines
• Self-review completed
• No new linting issues
• Changes verified locally
  ================================================================================